For deep drawing applications, prior art workers have produced both rimming and aluminum killed steels having a conventional manganese content of about 0.27% to about 0.40%. Rimming steel is cheaper to manufacture and has cleaner surface properties in ingot form and as rolled. A small amount of temper rolling after annealing will eliminate as-annealed yield point elongation (YPE), but the steel will still age at ordinary room temperature (about 23.degree. C.) in about 2 months resulting in the return of objectionable yield point elongation. Aluminum killed steel, on the other hand, will be permanently non-aging after a small amount of temper rolling following an anneal, so long as it is not exposed to elevated temperatures after the cold working. However, the non-aging quality of aluminum killed steel can be destroyed if the steel is subjected after temper rolling to a temperature as low as about 400.degree. F. (205.degree. C.).
As is well known in the art, the performance of sheet steel during deep drawing can be reasonably accurately predicted from the average plastic strain ratio, r.sub.m. An average r.sub.m value is normally obtained from tensile tests on several specimens most usually taken at 0.degree., 45.degree. and 90.degree. to the rolling direction of the samples. The r value in each test direction is taken as the ratio of the width strain to the thickness strain. The average plastic strain ratio is then computed by the formula: ##EQU1## Rimming steels with conventional manganese content from about 0.27% to about 0.40% demonstrate an r.sub.m of about 1.2. Aluminum killed steels having the same conventional manganese content usually demonstrate an r.sub.m of about 1.6. With both types of drawing quality steel, the hot reduced and cold rolled product is subjected to a box anneal. The box anneal for conventional killed steels is so conducted that the coldest temperature of the critical coil (usually the bottom coil in a single stack array) exceeds 1280.degree. F. (693.degree. C.). The prior art recognized that for conventional killed steels, r.sub.m is a function of temperature and soak time. An exemplary prior art anneal cycle for conventional killed steels has been about 1300.degree. F. (704.degree. C.) or more, with a soak time of 16 hours or more.
More recently, prior art workers have turned their attention to low manganese rimming and aluminum killed steels, having a manganese content of up to about 0.24%. With such low manganese rimming and aluminum killed steels, the steels have been subjected to substantially the same steps of hot rolling, cold rolling, annealing and temper rolling as were the conventional manganese rimming and aluminum killed steels. Prior art workers generally accepted that the cold spot in a box anneal should exceed about 1280.degree. F. (693.degree. C.). A typical standard practice box anneal cycle for low manganese, aluminum killed steel has been 1300.degree. F. (704.degree. C.) with a soak time of 16 hours, resulting in a cold spot of at least about 1280.degree. F. (693.degree. C.). Low manganese rimming steel has demonstrated r.sub.m values of about 1.5, while low manganese aluminum killed steel has demonstrated r.sub.m values of at least 1.7.
U.S. Pat. No. 3,668,016, for example, teaches a core-killed steel having a manganese content of from about 0.04 to about 0.02%. The reference speaks of box annealing at 1290.degree. F. (700.degree. C.) or 1310.degree. F. (710.degree. C.) with a soak time of from 4 to 5 hours. U.S. Pat. No. 3,709,744 teaches a vacuum degassed steel having a manganese content of 0.15%. This reference teaches an annealing temperature of from about 1200.degree. F. (659.degree. C.) to about 1350.degree. F. (732.degree. C.), followed by a soak of at least 12 hours. The preferred annealing practice according to this reference is a soak at about 1300.degree. F. (704.degree. C.) for a minimum of 12 hours and preferably for about 20 hours. U.S. Pat. No. 3,239,390 teaches a low manganese aluminum killed steel for enameling. The reference speaks of annealing at a temperature of 1290.degree. F. ( 700.degree. C.) with a soak of 5 hours. All of these references are exemplary of prior art low manganese steels subjected to conventional anneals.
In recent years manufactures have offered a deep drawing, aluminum killed, conventional manganese steel which is pre-painted and supplied in coil form by the manufacturer prior to fabrication by the customer. The coiled painted strip is cured by baking at a temperature of at least about 400.degree. F. (214.degree. C.) and usually at 490.degree. F. (254.degree. C.). Because of its aging characteristics rimming steel cannot be offered in a prepainted form. Even the aluminum killed, pre-painted, conventional manganese steel is subjected to a large number of rejects as the result of strain lines during subsequent forming. These strain lines are caused by aging during paint baking following temper rolling and are related to the presence of agglomerated carbides, nitrogen pick-up, or both.
The present invention is based upon the discovery that the r.sub.m value for low manganese, deep drawing, aluminum killed steel, unlike conventional manganese deep drawing aluminum killed steel, does not improve with annealing temperature and/or time. In fact, with the low manganese aluminum killed steel, virtually the maximum r.sub.m value is obtained immediately after complete recrystallization. As is known, lowering the manganese content also lowers the recrystallization temperature. Thus, the higher temperature and soak time of a conventional box anneal for a conventional manganese, deep drawing, aluminum killed steel, when applied to a low manganese, deep drawing, aluminum killed steel, does not improve the r.sub.m value, but rather promotes unwanted grain growth, nitrogen pick-up and agglomeration of the carbides. These results tend to promote aging and strains in the metal upon the forming thereof. Unwanted grain growth can produce orange peel strain (rough surface) upon forming, which may be objectionable.
It has further been discovered that excellent r.sub.m values can be achieved when a low manganese, deep drawing, aluminum killed steel is box annealed in such a way as to achieve a cold spot temperature of at least 1100.degree. F. (593.degree. C.) and less than 1250.degree. F. (677.degree. C.). Ideally, the innermost and outermost convolutions of the coil should not exceed 1330.degree. F. (721.degree. C.). No soak time is required.
This box annealing treatment has a number of advantages. The lower temperature anneal produces excellent r.sub.m values and no serious abnormal grain growth problems occur which were previously found to be characteristic of low manganese, aluminum killed steel. Carbide agglomeration and nitrogen pick-up are are greatly reduced or eliminated. Productivity is increased by 30% or more (tons per hour) while achieving a savings in both energy and annealing gases used.
Furthermore, aluminum killed, low manganese steel, processed according to the present invention will not age when subjected to heat treatments up to about 550.degree. F. (288.degree. C.) and therefore is excellent for use in the manufacture of a pre-painted product.